Mechanical element which is to be subjected to high temperatures



July 12,1949.` H. sco-rr ErAL MECHANICAL ELEMENT WHICH IS 'TO BE SUBJECTED TO HIGH TEMPERATURES Filed Sept. 29, 1.944

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Patented July 12, 1949 UNITED STATES PATENT CFFICE MECHANICAL ELEMENT WHICH IS TO BE SUBJ ECTED T HIGH TEMPERATURES Howard Scott and Robert B. Gordon, Pittsburgh,

Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 29, 1944, Serial No. 556,385

2 Claims.

`been generally considered as superior alloys, the

chromium imparting oxidation resistance while the nickel tends to maintain the alloys austenitic, that is, having a face-center-cubic crystal structure.

The known austenitic iron-nickel-chromium alloys, however, do not have high strength or creep resistance at elevated temperatures in excess of 1200" F., the strength rapidly diminishing as the temperature is raised. None of the commercially available austenitic iron-nickel-chromium wrought alloys have the characteristic of being resistant to creep and rupture at temperatures of 1500 F. or higher under stresses of 10,000 to 15,000 pounds per square inch for periods of time in excess of 1000 hours.

An object of this invention is to provide a precipitation hardened austenitic alloy which is resistant to creep at elevated temperatures.

Another object of this invention is to provide an austenitic alloy containing nickel, cobalt, chromium, molybdenum and tungsten which can be formed as by machining or forging to a predetermined shape and precipitation hardened to impart thereto resistance to creep at elevated temperatures.

A more specific object of this invention is to provide a low carbon alloy containing nickel, cobalt, chromium, molybdenum, tungsten, manganese and silicon so balanced that the alloy can be precipitation hardened to a hardness in excess of 300 DPH while imparting thereto resistance to creep at elevated temperatures;

Other objects of this invention will become apparent from the following descriptionwhen taken in conjunction with the accompanying drawing, the single ligure of which is a graph, the curves of which illustrate the relation of `a precipita- (Cl. 14S-32.5)

2 tion hardening coefcient for the alloys of this invention to the hardness and creep rate obtained with the alloys.

We have found that alloys of the heat resistant type can be produced having oxidation resistance, high strength and resistance to creep at elevated temperatures of 1500 F. and higher, the latter characteristics being imparted to the alloys by a precipitation hardening treatment. The alloys contemplated within the scope of this invention, in general, are low carbon alloys containing balanced amounts of nickel, cobalt, chromium, molybdenum, tungsten, manganese and silicon, the alloys being precipitation hardened by solution treating them at a high temperature and thereafter ageing them at a somewhat lower temperature to give the alloys high strength at elevated temperatures.

In particular, the heat treatment which has been found to be suitable for developing the characteristics of the alloys of this invention comprises quenching them from a solution temperature between 2200 F. and 2375" F. at which they are soaked for a period of time of 0.5 to 4 hours `and ageing them at a temperature between 1500 7 F. and 1700 F. Generally, the period of time of ageing the alloy depends upon the treatment. Practically, however, the ageing temperature for any particular alloy is usually selected to produce maximum hardness in a particular time. Thus a time of Ll0 to 50 hours at an ageing temperature of 1550 F. or a time of 20 hours at an ageing temperature of 1600 F. is satisfactory for developing the characteristics of the alloys of the present invention.

The proportions of the constituents of the alloy may be varied over wide ranges in a predetermined manner as long as the alloying constituents are balanced as will be explained in full hereinafter. In general, the alloy composition may range from 15% to 35% by weight of nickel, i

20% to 40% by Weight of cobalt, 17% to 22% by weight of chromium, 3% to 15% by weight of molybdenum, 1% to 9% by weight of tungsten, 0.5% to 3% by weight of manganese, 0.1% to 0.7% by weight of silicon, not over 0.20% by weight of carbon and the balance iron with not `more than 0.5% by weight of the usual contaminants such as impurities and deoxidizers.

It is preferred to maintain the carbon content of the alloy at less than 0.20% and preferably lower than 0.12%. This is because carbon has been found to form carbides with chromium, molybdenum and tungsten and thereby lowers the oxidation resistance of the resulting alloy, the carbon having the further disadvantage of detracting or interferring with the precipitation of the precipitation hardening compounds to lower the resulting hardness of the alloys.

In the alloys of this invention, the nickel. and cobalt .contents are interrelated lcooper-ating to control.thesolidfsolntion hardening of the alloy, the lower limit of 20% cobalt being utilized for increasing the ductility of the alloy and improving,

its forgeability. It has been foundthat by substituting cobalt for nickel, a reduction in the amount of the hardener elements necessary for obtaining maximum strengthnswillbe .evident hereinafter, is obtained with anaccompanying increase in forgeability. Where the lower ,limit of at least 20% cobalt is utilized, it is necessary to employ at least niclgehin order to maintain the alloy austenitic.

The chromium is primarily utilized for im parting corrosion and oxidation resistance to will@ @Whitt-beine lllldthatnot-,less than 17% chromium is necessary .i i-the vpresent yalloy for giving the required corrosion andoxidation resistance to the .a1i1oy,=which will permit its use at .temperatures M1500." .-F. and higher. As` the chromium content isdncreased beyond the micheland cobaltccntentsalsohave to be increased Ato 4maintain solution hardness require- .ments as willbeapnarent from the` relation of ,the alloying. ,elementsy in maintaining a, balanced relation4 as .givenhereinafter andit is therefore ,preferred to utilize 4notnver y22% chromium in .thepresent alloy.

,In order to .insurethatthe present ,alloy is .susceptible-to :a precipitation vhardening treat- -mentlfr.om`3 vto 15% `lof .molybdenum and from 1% to 9% of tungsten .areutilized it being found that these elements when properly balanced with- ;in the rangesagi-ven,@together.with a proper bal- Lance .ofthe `other alloying constituents, as will be explained .more :fully hereinafter, are very vact-ive Vprecipitation hardening agentsin the resnltingaustenitic alloy. While molybdenum and tungsten constitute l.the active.. precipitation hardlening agents, it has alsov been found that the iother alloying,elementshave a definite effect on the characteristics of the alloy, Yand that the relation orvbalance of thel alloying constituents in :the all-oy must be rigidlyl controlled, as will be explainedhereinafter. v

yIn introducing the elements, nickel, cobalt, `chromium, molybdenum'andftungsten, pure metials may bev usedifso desired, or the ferro-alloy fform of the valloying constituents may be used.

Care should be exercised, however, in order not *.toLeX-ceed' 251% iron asa maximum in the alloy if tlieferro-alloys are employed whereas if commercially pure nickel, cobalt, chromium, molybdea'ndftung-sten is' employ-ed `only that amount of' iron, amountingto a few tenths of a percent,

- which is. present in such metals, is present in those alloys within the range given hereinbefore whose alloying constituents have the relation that and (2) %Mo+0.75(%W)=10 to 15 are satisfactory.

We have further established that in order to respond in Satsfactorydegree to the precipita- %Ni+0.5(%CO) =30 to 47.5

tion hardening vtreatmentfreferredjto hereinbefore, the alloying constituents must be so balanced as to have the relation that 'Wlezha-ve found'that when a value lower than 19 `is obtained ywhen the alloying constituents within the ranges given are substituted in the foregoing equation, the alloys Will lack hardenability and ,if the value .ofthe alloying constituents when substituted in Equation 3 give a value greater than 24, the alloys are deficient in forgeability. For convenience the quantity calculated by substituting l the Vvalues of the f alloying constituents l in -thel-foregoing. equation is termed the precipitation hardening. Acoemcien-tf- .or -PH-C .since such Aquantity has been found 4to vary Vdirectly with .the agedh-ardness oftnealloysy within the range of .the composition .given .hereinbefore From YEquation 3 .it Vis seen l.that molybdenum, chro- :mium,tungsten, manganese and silicon, having a Vpositiv-e sign, actgto increase the yprecipitation hardening lcoefficient and hardness, whereas .the :nickel and cobalt: act to decrease theprecipitation hardening coefficient and-hardness.

-We have 'found that -the solution hardener content of the alloy-has `-a `:defi-nite effect in imparting high strengthto. the-alloy, renderingl the alloy suitable lforfusea-t-temperatures of 1500'F. or higher. On the basis of our investigations we `have derived an empirical equation termed for convenience the "vc-oecient for solution hardening orffSHC whch'the allo-ying* constituents of our alloys must satisfy in order to have adequatecreep strength. This relation among the alloying constituents is setvforth in the following equation Equations .3 and 4, thealloysfof this invention will not-.have adequatecreep resistance unless cipitation hardening coefficient and the coef.-

.cientfor'solutionihardening andare heat treatled lby quenching and ageing them Within the rangesigiiven, it `hasrbeenfound that the characvteristic's ofl-the alloys can be readilyduplicated numerical value obtained by substituting the va'lue of ftheall'oying constituentslin `Ecuiati'ons 1,:2, y3 and -4 given lhereinbefore.

Value of Equations 2.0.0.L22ZZLL2LLLLLLL Table I MoW Mn Composition, Wgt. Per Cent 0.0.0.0.0.%.0.0.0.0.0.0.0.0.9.1nwm 23332 223333332332 Alloy As will be noted, a number of the alloys given in the foregoing table fail to meet one or more of the relations established by the empirical equations given hereinbefore. In the foregoing table it is, of course, understood that the balance of 25 each of the alloys listed constitutes iron with less than 0.5% of the deoxidizers, such as aluminum, titanium, vanadium and calcium and impurities such as phosphorus, sulphur, arsenic, tin and copper.

Referring to the following table, the vheat amm maw. ah Tew nh .ES .lfS doe e n tSd ser Hua. l. S Snahy V.V 0 MGMH .mihtawaJ ehdn hwhee .E tr. Wa@ iwre emi. dh d mamme Debit Dimm a ln tdoo es nod d rse man ..0 .1 ...asa eb n. nO

loys M284, M492, M519 and M537 which have a hardness in excess of 300 DPH gives far superior creep resistance over that obtained on alloys M440, M465 and M481 which have DPH values lower than 300. However, by properly selecting the proper heat treatment within the range given hereinbefore, the hardness of the latter alloys can be raised to values over 300 DPH and creep resistancevalues comparable to those obtained on alloys such as M492 can be obtained.

The results of the creep rupture tests given in Table III are based on alloys of approximately the same matrix composition, the content of the hardener elements, however, varying over a considerable range as shown by the values of the precipitation hardening coeflicent listed for cipitation hardening coefficient on the hardness and on the creep rate after a standard heat treatment is clearly shown by curves I0 and l2 respectively, of the drawing, these curves reprecipitation hardening coeicient as given by Equation 3 results in an increase in the hardness and a decrease in the creep rate at 20,000 pounds per square inch at 1500o F.

As a specic example of the creep rate obtained Iwith an alloy of this invention, an alloy having a composition of 19.98% nickel, 30.1% cobalt, 20.23% chromium, 8.32% molybdenum, 3.75% tungsten, 1.97% manganese, 0.25% sili- 0.11% carbon and the balance iron with not more than 0.5% impurities was cast into a 6" square ingot weighing 270 pounds. It was found that the alloy thus cast was readily fab ricated having excellent forgeability and good Referring to the following 75 oxidation resistance in air up to 2100 F. While By referring to Table II it is apparent that al- Equation 3 in Table I. The effect of the pre- DPH Time, Hrs.

Ageing Treatment H e .70. a T

Time, Hrs.

Solution Treatment Temp.,

The alloys listed in Table I and falling within the range of 19.5% to 21.5% by weight of nickel, to 31.5% by weight of cobalt, 19.5% to 21.5% by weight of chromium, 8% to 9.5% by G5 Weight of molybdenum, 3% to 5% by weight of by weight of carbon and the balance iron with not more than 0.5% by weight of impurities 70 con when precipitation hardened as given hereinbe- Alloy No.

2, 3 and 4 will have a minimum hardness of 300 DPH such as is required in the preferred spectiveiy inustrating that an increase in the alloys of this invention.

tungsten, 1.5% to 2.2% by Weight of manganese, 0.1% to 0.4% by weight of silicon, less than fore is of particular value for use in turbine blades or the like, as such alloys are exceptionally resistantto creep at elevated temperatures of 1500" F. or higher.

From an examination of Tables I and II, it is apparent that only those alloys which satisfy the relation set forth by the empirical Equations 1,

ausge@ being machinable. When this alloy was subjected to a precipitation hardening heat treatment consisting of a solution treatment for 4 hours at 2350 F. and then quenched in eilandY s them unusually lonen life in industrial applications.

Although this invention has been described with reference to a particular embodiment'therethereafter aged for 240 hours at 1500 F., ahard- 5 oi, it` is, of course, not to. be limited thereto exness of 370 DPI-I was obtained. The outstanding' cept insofar as is necessitated by the scope of creep and rupture properties for this alloy are the appended claims. listed in the following table: l We claimas our invention:

Table IV l. A mechanical element' whichis to-be subfl; jected to aitemperatureof1500" F; and higher' Minimum Timm and considerable'stress in normall usejconsist'# Creep Raie, Rliilre Rsltlrlfillllfe 111%v ing of aprecipitationhardened austenitic alloy' r. per'. pefcrtpef Hrs. percent sgg?, Vcorn-puree 0fs15% to 35% nieke1,20% 004017:, eo-

' halt, 17% to 22% chromium,l 3% to'15%moly'b 13: denum;` 1% tol 9% tungsten,l 0.5%i.to.3% mane i; gg Obi? ae] ganese, .0.1%.to 0.7% silicon, less than-0.2%. cari, ig, gggg 4,709+ bon, and the balanceriron with` less than 0.5% 1j 000 91400 Z000- III of impurities, Whichthas beeniquenchedziromra if solutiontemperature-'between-2200'F.'and 23751o go F., the solution' temperature bein'g'm'aintained .eS-@Example @the tenere meer Obtained dii flinmpittte ioi withalloys of thisinvention atdiierent tem- 7009 F for, a eriod of ifm between h ur peratures ranging between.y room temperature y p 1 e o. s D y and 240 hours, the alloy having the relation and 1600 F., the following table lists the tensile m, n t u t, th t* properties obtained 'on theialloy M537 identified 25 a o g 1 sa Oymg componen s a in the foregoing tables and gives the heat treat- %Mo+0.8(%Cr) +0.l75(%W-{%Mn)-|-1.2(%Si) ment identiedrtherewith in Table II. -0.25(%Ni) -0.125(%Co) =19 t0 24 Table V 'reet initial Yieia Uitl-mare nieege- `lRei in. Spec.No. Temp., Hardness, Point, Strength, -tio e Area,

F. DPH p.'s:1. p.s. 1. j percent percent 70 334 87,1000 131,000 3 s 1,000 i340 77,000 113,400,` 9 9 i, 200 343 72,000 111,000 11 12 v1,350 330 59, 000 87,700`v 20 i9 1, 500 Vi 47, 000 09,400 3o s2 1,000-y iss i 4i, 000l 4s,=i00 as 29 v'Illie'speciinens A through' Fidentied inA Table the precipitation hardened alloy having a hard- V5 were yof 01200l in diameter overa 1l gage ness in excess of 300 DPH.'

and. Were -'-f0r'20'-h`0urS' at 2 A mechanical' (zjiznjnf,A is to be Sub.. for'tlfiev testsat 1350"'F.' or lower andior- 2i)y eel-,ed io e temperature of15009` F. and higherhours at tst'tmp'erature vfor the testsat 1500 and considerable stress in normali use consist- Ffand 1600" F. ing of a` precipitation hardened austenitic alloy The alloys of'-- this invention are austenti', composed 0f 19.5%'` to 21.5% nickel, 29% toy thatl is, they` have a faceecenteredecubic. crys- 31,5% cobalt,4 19.5% to 21.5% chromium, 3% i-,ov tal structure arifdli'ave exceptional'corrosionxre 95% molybdenum, 33% io,.5-% tungsten, 1 5%l;.o; sist'aifr'ce and high 'strengthiand creepresistan'ce 252% manganese, 0 1% i-,o :0.4% silicon, lees than at elevated temperatures.. 'hefallOYSj Whos@ '221+ 50 0.20% carbon,. and' the balance iron with notloyine constituentahave affrelationwhi'h: will more than 0.5% of impurities, which has been Satisfy the fOmuJaS- vgillen hellenbefoe- 'and quenched from a solution temperature between whichfhav'e a DPI-Inf 300 Orglea'teandigrain 2.200o F. and 23'75" F., thewsolution temperature si'z'e as coarse as-at-.leastNo.2.ASTM;are:parbeing maintained foia, period of time of loeticiilarl'if` applicable `fo'rfuse in'fgas'turhnes: where 55 tween 0,5 and 4 hours, and aged ei e, temperait? is necessary toemply anfalloyfhevinfe'rleture between 150015. and 1700 ii'. for a period tively high tensile strength-10W Creep-'rate and cfftime-between 20 hours and 240 hours, the a1- a .high degree oi.y oxidationf. resistance; Further; loylieviiig the relation among its ailoying ooi-n..` the alloys of this;` inventionl 2re paiwrltliculay 60 ponentsthat a'daptedifor` use as parts ofltur lines.I were -1Ae i temperature gradients-:range fromth'eimaximum' %M0+U8(%Cl 2'0'75(%W0`*'1%Mn) +1'2(%S1)' down to as low as .room `temperature fandtwhere (%N1) 25(%C)=19 to 24 the .turbine parts .may` bei subjected to stresses thefprecipitatioii hardened alloy having a hardwhich vary immenselyA with the.` temperature; ness inexcess of 300 DPH. Other uses of thesev `alioys: areas.-r articles of HOWARD SCOTT. manufacture such: as lbol'ts, springs ori-'the like' ROBERTi B. GORDON. where high' 'stressesv ati elevatedfA 'temperatures may be encountered. '-Iflieis'e-alloys` are'particu-nA ERENCES CITED- 121137 useful as they; may 'bei readily fOr-medi@ Thefollofwiirg referenloes are of recordin` the shape as by forging'or niechanicaliggffiivorkin'lg1 anu 70 fue of this paient; thereafter subjected to=thef precipitation: arde eningtreatment 'to imparti the required" charac'e UNITED STATES'PATENTS teristics- -thereto. Further, thealloys oitihijsiiie Number Name Date Veniion .li-eve unusual:thermal@stability'etvopere v Re. 20g-877 Prange 'Octi 4,' 193`8`^` ating temperaturescf'lf iorhigillenelif-ing "T5 i ftotherireerencesfonfollowing page) Number Rohn June 17, 1941 FOREIGN PATENTS Number Country Date 698,724 France Feb. 3, 1931 5 OTHER REFERENCES Age Hardening of Metals, Symposium 21st Annual Convention of American Society for Metals, Cleveland, Ohio, Oct. 1939, pages 5, 

